1. Field of the Invention
The present invention relates to a dynamic pressure bearing device which can generate a dynamic pressure upon relative movement between a shaft and a bearing in either a forward or a reverse direction.
2. Description of the Prior Art
Conventionally available dynamic pressure bearing devices of such a type as described above include one as shown in FIGS. 4 (a) and (b) (Japanese Utility Model Publication No. 12971/1991). FIG. 4 (a) is a cross sectional view in the axial direction of the dynamic pressure bearing device, and FIG. 4 (b) is a development view showing a sliding surface of the inner circumference of a bearing in the dynamic pressure bearing device.
This dynamic pressure bearing device has a column-shaped shaft 52 partly accommodated in a cylindrical bearing 51. Although not shown, oil as a lubricating fluid is charged between the bearing 51 and the shaft 52. Also, a plurality of rhombus dynamic pressure grooves 55 are provided to a sliding surface 53 of the inner circumference of the bearing 51 confronting the shaft 52, as shown in FIG. 4 (b).
Each of the rhombus dynamic pressure grooves 55 is positioned on the sliding surface 53 in such a manner that, of two orthogonal line segments L and M formed by joining the opposing vertexes of the rhombus dynamic pressure groove 55, one line segment L extends in the circumferential direction of the shaft 52 while the other line segment M extends in the axial direction of the shaft 52.
With the above dynamic pressure bearing device, when the shaft 52 rotates in a rotating direction A as shown by an arrow A in FIG. 4 (a), corner portions 60 of the dynamic pressure groove 55, which correspond to the rotating direction A, cause the oil between the bearing 51 and the shaft 52 to generate a dynamic pressure, thereby supporting the shaft 52 in the radial direction. On the other hand, when the shaft 52 rotates in a rotating direction B as shown by an arrow B in FIG. 4 (a), corner portions 70 of the dynamic pressure groove 55, which correspond to the rotating direction B, cause the oil between the bearing 51 and the shaft 52 to generate a dynamic pressure, thereby supporting the shaft 52 in the radial direction.
However, the above-described conventional dynamic pressure bearing device has had the following problems.
(1) When the shaft 52 is reversed in its rotating direction, the flow of the oil within the rhombus dynamic pressure grooves 55 will also be reversed. In this case, the oil is required to abruptly change its flowing direction at corners P and Q at which the corner portions 60 and 70 of a dynamic pressure groove 55 join together, as shown in FIG. 5 (a), which is a schematic view of the rhombus dynamic pressure groove 55. This can cause a less smooth oil flow, with the result that the dynamic pressure generated is likely to fluctuate.
(2) When the shaft 52 moves linearly in the axial direction while rotating, there may occur a case in which the direction of movement of the shaft 52 with respect to the dynamic pressure groove 55 becomes approximately orthogonal to grooves 60a and 70a which form two opposite sides of the rhombus dynamic pressure groove 55, as shown in FIG. 5 (b), which is a schematic view of the rhombus dynamic pressure groove 55. In this case, flow of the oil within the grooves 60a and 70a orthogonal with respect to the above-noted direction of movement would stagnate, fail to generate the dynamic pressure.